1855.] 



MICRO-CHEMICAL EESEAECHES ON THE DIGESTION OP STARCH, &e. 



311 



By these experiments it was determined tliat the concentric circles 

 remain after the whole of the starch matter, colourable by iodine, was 

 removed, and that even then the characteristic cross and colours were 

 still seen when the granules were viewed by polarized light, although 

 more feebly than before ; this result being probably due to the lessen- 

 ed power of refracting light, after the removal of the starch matter. 



After describing the structure of the wheat-graiu and flour, the 

 changes occurring in the wheat-starch during the manufacture of 

 bread are given in detail ; but the most interesting of the changes pro- 

 duced by cooking are those seen in the boiled or roasted potato and in 

 the boiled jea. 



In each of these the act of cooking effects two purposes : — it causes 

 great enlargement and physical change of the starch-granules, and dis- 

 solves the intimate adhesion of the starch-cells, which afterwards ap- 

 pear as ovoid or globular, slightly adherent bodies distended by the 

 swollen starch-granules, the out-lines of which are indicated by more 

 or less irregular gyrate lines, produced by the mutual compression of 

 the starch-granules within an inelastic cell-membrane. 



The starch-granules of the- pea possess a much thicker investing 

 membrane than those of the potato, which causes their outlines to re- 

 j main much more distinct after the removal of the true starch substance 

 during the process of digestion. The other structures seen in the pea 

 are carefully described; the most curious among them being the cells 

 composing the external layer of the testa, which bear so strong a re- 

 semblance to columnar epithelium of the intestine, that they might be 

 mistaken for the latter by an inattentive observer. 



The substances submitted to experiment were,^l, boiled wheat- 

 starch ; 2, wheaten bread ; 3, uncooked tons les mois ; 4, boiled tous 

 les mois ; 5, boiled potato ; G, uncooked peas ; 7, boiled peas ; 8, boil- 

 ed peas after ligature of the bile-duct ; 9, boiled potatoes after lig- 

 ature of the bile and pancreatic ducts. Several subsidiary experiments 

 were made to determine the action of the intestinal mucus, the saliva, 

 and the substance of the pancreas, on starch. 



The conclusions at which the author arrives from the experiments 

 are,^ 



1. That the slarch-granule is composed of two parts, chemically and 

 histologically distinct, — a cell-membrane and homogeneous contents. 

 The markings seen on many varieties of starch are referred to folds or 

 markings of the investing membrane. 



2. No perceptible change occurs in the starch, whether raw or cook- 

 ed, during its sojourn in the stomach of quadrupeds or the ventrieulus 

 succenturiatus and gizzard of birds ; all the granules preserve their 

 perfect reaction with iodine and their pristine appearance. 



3. The conversion of boiled starch into dextrine and glucose is chiefly 

 effected in the first few inches of the small intestine, but it continues 

 to take place in a less degree throughout the entire intestinal canal. 



4. Ill the digestion of boiled wheat or other starch, or of wheaten 

 bread, the bulk of the mass rapidly diminishes in its passage through 

 the small and large intestines, so that it ultimately yields only a small 

 quantity of fa;cal matter. After being deprived of their contents, the 

 membranes of the granules shrink and shrivel tip into a minute granu- 

 lar matter, which constitutes the chief bulk of the fiscal evacuations 

 after an exclusive diet of starch food. 



6. The digestion of raw starch food (peas) in the pigeon or other 

 granivorous birds goes on much more slowly, and progresses pretty 

 equally throughout the entire intestinal canal. The starch-granules, 

 whether free or included in cells, become intersected by radiating or 

 irregular lines or fissures, more or less opaque or granular ; they also 

 gradually lose their characteristic reaction with iodine ; and this im- 

 portant change, commencing at the surface, progresses towards the 

 centre, until the whole of the starch matter is removed, leaving the 

 starch membranes often apparentlj' whole, retaining their characteris- 

 tic markings. The fissured and granular condition of the starch-gran- 

 ules is not due to their trituration in the gizzard, but to the action of 

 the intestinal fluids, since it was often seen in granules enclosed in and 

 protected by perfect .'^tarch-cells. In the digestion of raw starch food, 

 a considerable quantity always escapes change, for many starch-cells 

 and granules in the fasces perfectly retain the characteristic reaction 

 ■with iodine. 



G. As the starch remains unchanged in the stomach, its conversion 

 into glucose cannot be attributed to the saliva or gastric juice, unless 

 we suppose these fluids to remain inactive in the stomach, and sudden- 

 ly to regain their activity in the first part of the small intestine. The 

 author foiind that the saliva was capable of eft'ecting the conversion 

 of starch into glucose, but that the mixture of saliva and gastric juice 

 in the stomach did not possess that property even after being rendered 

 alkaline by carbonate of soda. It is probable that the converting power 



of the saliva, as it flows from the mouth, depends not on the true sa- 

 liva, but on the buccal mucus; for Magendie found that saliva taken 

 from the parotid duct was wholly inactive, while the mixed saliva from 

 the mouth effected the conversion with great facility. Unless, then, 

 the sublingual and submaxillary glands secrete a different fluid from 

 the parotids, it is evident that the activity of the saliva must be attri- 

 buted to the buccal mucus. 



7. The difference between the digestion of hoUed and raw starch in 

 dogs is seen in the experiments on the digestion of boiled wheat-starch, 

 boiled tous les mois, and bread. In all these, some starch-granules 

 escape the action of heat and water, and remain in nearly their pristine 

 condition. These uncooked starch-granules undergo slow and imperfect 

 changes, being fissured, broken, and more or less altered, but in general 

 retaining their characteristic reaction with iodine. 



8. The conversion of starch into glucose is not effected by the bile, 

 since after ligature of the common bile-duct, the changes occur to as 

 great an extent as when the bile passes freely into the intestinal canal. 



9. It is not due to the pancreatic juice, inasmuch as after ligature 

 of the bile and pancreatic ducts in the same animal, the digestion of 

 starch is still effected. 



10. The only remaining secretion is the intestinal mucus, which is 

 especially abundant at the upper part of the intestinal canal : and a 

 further proof is afforded of the activity, of the intestinal mucus taken 

 from the upper part of the duodenum above the entrance of the pan- 

 creatic duct after ligature of this duct and the common bile-duct, by 

 its capability of converting a large quantity of fresh boiled starch into 

 glucose out of the body. 



11. In the cooking of starch-containing vegetables, such as potatoes 

 and peas, the adhesion of the starch-cells'is dissolved or weakened, so 

 as to render them easily separable and amenable to the action of the 

 intestinal fluids. At the same time. the starch-granules undergo a 

 large increase in bulk, distend the cells, and by their mutual compres- 

 sion, their outlines present th(i appearance of gyrate lines beneath the 

 cell-wall. The cells seldom burst so as to emit their contents, or pre- 

 sent any appreciable opening through which the intestinal fluids can 

 directly penetrate. The author cannot positively a£5rm so much of 

 the starch-membranes, because these are so extremely delicate that 

 fissui-es might be invisible, but he believes that in a great number the 

 membranes remain entire. 



12. If this be the case, the conversion of starch matter into glucose 

 must be effected by the permeation or endosmose of the intestinal fluids 

 through the invisible pores of two membranes, in the digestion of the 

 pea, the potato, and other similar foods, and the glucose must escape 

 through the same membranes by cxosmose. 



13. Before the conversion of starch into glucose, the amylaceous 

 matter contained in the starch is more dense than the intestinal mucus 

 in immediate contact with the cells, and an inward current or endos- 

 mose is established ; but after that conversion, the syrupy fluid is 

 less dense than the mucus, and then an outward current or exosmose 

 occurs, by which the glucose escapes from the cells into the intestine 

 and is absorbed. If this be the case, as the details of the experiments 

 tend strongly to prove, a new and important function is assigned to 

 the intestinal mucus. 



14. In normal digestion, chyme escapes very slowly from the stom- 

 ach into the duodenum, in small quantities, as it is detached from the 

 alimentar}' mass by the muscular movements of the stomach, and this 

 gradual propulsion often occupies several hours after a meal. This 

 slow proptdsion is evidently intended to expose the comminuted food 

 fully to the action of the intestinal juices, and produce an intimate 

 mixture with them. The comparatively empty condition of the upper 

 pai-t of the small intestine, even during active digestion, is thus fully 

 explained. 



15. If the food be too finely divided or incapable of a second solidi- 

 fication in the stomach, it passes too rapidly into the first part of the 

 small intestine, is insufliciently mixed with the intestinal fluids, and a 

 considerable part escapes digestion. On the other hand, if it enters 

 the small intestine in masses incapable of reduction by the muscular 

 action of the parts or solution in the fluid, fit traverses the intestinal 

 canal unchanged, except at (he surface, which is then alone exposed 

 to the action of the intestinal fluids. 



IG. It is not necessary for the conversion of starch into glucose that 

 the fluids in the duodenum or other parts of the intestinal canal should 

 be alkaline, or even neutral, for in several of the experiments the eon- 

 tents of every part of the alinientary canal had an acid reaction. 



17. The greater part of the intestinal mucus is not excrciiienlitious, 

 for little, if any, mucus is perceptible in the fieccs in normal digestion, 

 except at their surface, whereas the greater proportion of the contents 



